1. Field of the Invention
The present invention relates to a method for fabricating a grating coupler used for an optical communication system, and more particularly a method for fabricating a grating coupler having an epitaxial layer of which surface has a cross-hatch pattern that serves as a grating, in which the epitaxial layer is grown by an epitaxial growing process using a material having a lattice constant greater than that of a substrate or an atmosphere.
2. Description of the Related Art
In general, a grating coupler is an optical coupler fabricated by artificially forming the grating on a material used for a semiconductor or dielectric optical waveguide, so as to transfer an incident beam from the surface through the optical waveguide. A known method for fabricating the grating grows a thin film on a substrate to be used for the waveguide, and forms the grating on the thin film by the masking and etching process. However, in the case where a spacing between the gratings is very narrow, an optical interference exposure process has been used for fabricating the same.
Referring to FIG. 1, there is shown a cross-sectional view of the grating coupler according to prior art. As illustrated, an optical waveguide layer 2 is formed on a substrate 1. Then, a plurality of gratings 3 are formed uniformly on the optical waveguide layer 2 by the conventional masking and etching process. It is generally noted that a material forming the optical waveguide layer 2 should have a greater reflective index than the reflective index of the substrate 1. Furthermore, a periodicity .DELTA. of the gratings (i.e., a spacing between the gratings) and an angle .theta.m of the incident beam should be properly evaluated, in order to effectively guide the optical beam.
As described in the foregoing, however, the prior art method for fabricating the grating coupler includes a complicated manufacturing process such as the masking and photo lithography, and requires a long manufacturing time. In particular, therefore, in case of fabricating the grating coupler into a small semiconductor chip, such prior art method including the masking and etching (or photo lithography) process is not proper.
Meanwhile, if the optical waveguide has a plane surface, the incident beam from the exterior can not be effectively guided within the optical waveguide. Thus, such optical waveguide having a plane surface is not suitable for the coupler. Therefore, a coupler including the grating on the surface thereof is one of the solutions proposed for settling the problem. Here, the term "coupler" used in the application refers to an optical element for transferring an optical beam propagating in a particular medium to another medium. In order to transfer energy from the optical beam to a particular waveguide mode accurately, a propagation constant in the particular medium should be identical to a propagation constant in a medium to which the optical beam is transferred.
As illustrated in FIG. 1, the incident beam partially reflects on the surface of the grating 3 formed on the surface of the optical waveguide layer 2, and partially penetrates into the optical waveguide layer 2 via the grating 3. Thus, in the case where the propagation constant in the grating 3 is identical to the propagation constant in the optical waveguide layer 2 and a critical angle .theta.m is below a particular angle, the penetrated incident beam propagates only within the optical waveguide layer 2. This relationship can be represented by, EQU .beta.=.beta.o+.upsilon.2.pi./.DELTA.
Where .beta. is a propagation constant of the optical beam within the optical waveguide layer 2 neighbored upon a lower part of the grating layer 3, .beta.o is a propagation constant at the optical waveguide layer 2 without the grating layer 3, .upsilon. is an integer, and .DELTA. is the periodicity of the gratings. Then, a propagation constant .beta.m at a medium having a reflective index n.sub.3 of the incident beam can be represented by, EQU .beta.m=kn.sub.3 sin .theta.m
where k is a propagation constant under vacuum which is identical to 2.pi./.lambda.o (.lambda.o is a wavelength of the incident beam under vacuum), .theta.m is the maximum incident angle. Here, when .beta. is identical to .beta.m, the penetrated incident beam propagates only within the optical waveguide layer 2.